Fluorinated polymers containing sulfonic acid functional groups, due to their ion conducting properties, have found widespread use in the manufacture of electrolyte membranes for electrochemical devices such as electrolysis cells and fuel cells. Notable examples are for instance proton exchange membrane (PEM) fuel cells which employ hydrogen as the fuel and oxygen or air as the oxidant.
Fluorinated polymers containing sulfonic acid functional groups have also been known to provide hydrophilic fluorinated surfaces due to the presence of the sulfonic acid groups.
To provide a high proton transport capability to an electrolyte membrane or to efficiently interact with water in a hydrophilic fluorinated surface polymers having a high number of sulfonic acid groups are required, which however are generally provided with reduced mechanical and physical resistance with consequent negative effects on the duration of the articles obtained therefrom.
The use of cross-linking to improve the physical resistance of membranes made from fluorinated polymers containing sulfonic acid functional groups has been previously disclosed. For instance, EP 1238999 A (SOLVAY SOLEXIS SPA) 11 Sep. 2002 and EP 1239000 A (SOLVAY SOLEXIS SPA) 11 Sep. 2002 disclose hydrophilic membranes comprising cross-linkable sulfonic fluorinated polymers comprising: monomeric units deriving from tetrafluoroethylene, fluorinated monomeric units containing sulfonyl groups —SO2F, and from 0.01% to 5% by moles of monomeric units deriving from a bis-olefin of formula R1R2C═CH—(CF2)m—CH═CR5R6 (wherein m=2-10, R1, R2, R5, R6, equal to or different from each other, are H or C1-C5 alkyl groups). The membranes are obtained by cross-linking of the sulfonic fluorinated polymer, the cross-linking involving the backbone of the polymer. The membranes are suitable both for use as ion conducting membranes in electrochemical cells as well as filtration membranes.
The use of bis-olefins of formula R1R2C═CH—(CF2)m—CH═CR5R6 suffers however from the drawback that, due to the low boiling temperature of the bis-olefin, fractions of the bis-olefin are lost by evaporation during the manufacturing steps of the membrane reducing the effectiveness of the cross-linking process.
Thus the need still exist for providing articles, in particular membranes, with improved physical and mechanical resistance by the cross-linking of fluorinated polymers comprising sulfonic acid functional groups without affecting the ion conducting capability and hydrophilicity of the polymer and with a process having improved efficiency.
It has now been found that the incorporation of azide-containing monomers into the fluorinated polymers comprising sulfonic acid functional groups provides a solution to the limitations of the prior art process.
The incorporation of azide-containing monomers in fluoropolymer chain has been described in the art, in particular for fluoroelastomers.
Thus, U.S. Pat. No. 6,365,693 (DUPONT DOW ELASTOMERS LLC) 2 Apr. 2002 discloses the incorporation of compounds of formula:CX1X2═CX—(O)p—Rf—(CH2)n—S(O)qN3 wherein: X, X1 and X2 are independently H or F, p is 0 or 1, n is 0-4, q is 1 or 2, Rf is a perfluoroalkyl or a perfluoroalkoxy group, as cure-site monomers in copolymers of fluorinated monomers, at least one of them being selected from VDF, TFE and chlorotrifluoroethylene (CTFE).
Similarly, US 2010324222 (DUPONT PERFORMANCE ELASTOMERS L.L.C.) 23 Dec. 2010 discloses a fluoroelastomer comprising copolymerized units of:                a first monomer selected from vinylidene fluoride and tetrafluoroethylene, and        a cure site monomer having a cure site selected from azide, sulfonyl azide and carbonyl azide groups.        
WO 2010/021962 A (3M INNOVATIVE PROPERTIES COMPANY) 25 Feb. 2010 discloses fluoropolymers containing one or more azide groups different from sulfonyl-azide groups.
None of these documents however disclose fluorinated polymers comprising sulfonic acid functional groups as well as azide functional groups.
US 2010093878 (E I DU PONT DE NEMOURS AND COMPANY) 15 Apr. 2010 relates to crosslinkable fluoropolymers, crosslinked fluoropolymers and crosslinked fluoropolymer membranes. In greater detail, the crosslinkable fluoropolymers disclosed in this document comprise:                from 70% to 95% by moles of recurring units derived from CR2═CF2 monomers, wherein R is H or F;        from 10% to 30% by moles of recurring units derived from CR′2═CR′—(O)q—RFSO2F monomers, wherein R′ is H or F, q is 0 r 1, with the proviso that when R′ is F then q is 1 and RF is a linear or branched C1-C20 perfluoroalkylene group, optionally containing oxygen or chlorine, and        from 1% to 8% by moles of recurring units derived from monomers having a vinyl ether end group, said monomers having formula CF2═CF—OR′F—(CH2)p—Z, wherein R′F is a linear or branched C1-C20 perfluoroalkylene group, optionally containing oxygen or chlorine, p is 0 or 1 and Z is selected from SO2N3, OCN and CN. Non-limitative examples of monomers of formula CF2═CF—OR′F—(CH2)p—Z include CF2═CF—OR′F—CH2—SO2N3, in particular CF2═CF—O—CF2CF(CF3)—O—CF2CF2—SO2N3. This document does not disclose or suggest sulfonyl azide- or azide-containing monomers having end groups other than vinyl ether end groups.        